Heat exchangers are commonly used in industrial processes. Typically, heat exchange systems are designed to transfer heat from one fluid to another. The design parameters are based upon the anticipated range of temperatures of the hot heat transferring fluid and the cold heat receiving fluid. In some cases, a series of heat exchangers may be in flow communication. In addition, in some cases, the temperature of the hot heat transferring fluid and the cold heat receiving fluid may vary quite dramatically depending upon the feed stock which is used in the process. Exemplary of such a situation is the contact process for sulphuric acid manufacture.
The contact process for sulfuric acid manufacture is commonly used to recover the sulfur values from gases discharged from metallurgical processes and from waste acid regeneration. For these operations, process heat from the oxidation of sulfur dioxide is transferred from converted or partly converted gases to the unconverted gases to heat the unconverted gases to reaction temperatures typically in the range of 400.degree. to 450.degree. C. In a double absorption plant, for example, there may be as many as six or seven heat exchangers in the exchanger train and the total exchanger area in the plant can be as large as 30,000 square meters. Gas flows can be as high as 200,000 normal cubic meters per hour and gas duct sizes can be as large as 2600 mm. Exchanger sizes may be as large as 6 meters in diameter and 25 meters high. A simplified schematic diagram of such an operation is set out in FIG. 1.
Where the gas source is a smelting furnace or acid burning operation, the gas strength produced may vary significantly and the design requirements of the various exchangers will vary in turn. The exchangers are each designed for the most difficult duty likely to arise and in other conditions, the exchanger performance will have to be decreased to satisfy the existing conditions. Such regulation of performance is normally obtained by passing a portion of the unconverted gas through external bypass means around the exchanger and mixing it with the gas passing through the exchanger at the exchanger exit. The mixed gas then passes to the next process operation. In some cases where the range of gas strengths expected in the acid plant is wide, almost total bypassing of some exchangers may be needed and the bypasses must be so designed. In other cases, there may be a need for very good mixing of the process stream before the next operation such as catalysis.
With the large gas flows and the modest pressure losses in the heat exchangers, the external bypass means has typically been a side stream around the exchanger with several changes of direction in addition to a bypass valve, a route with a significant flow resistance. The driving force for flow through the bypass line is the pressure loss through the exchanger. This driving force drops rapidly as the fraction bypassing the exchanger increases. When a high fraction of shell side fluid must bypass the exchanger, it may also be necessary to throttle the flow through the exchanger to create the needed bypass flow. Such throttling may be achieved by providing a valve on the inlet stream to the heat exchanger or by arranging the flow in such a way that the external bypass line is the preferred flow arrangement such as by placing the exchanger on the side stream and the bypass in the main flow stream. In addition, when the two gas streams are rejoined, there may be several hundred degrees differences in temperature between the bypassing stream and the main stream and mixing of the two streams is not automatic. Thus, the temperature of the recombined stream downstream of the exchanger may vary greatly from point to point. Such a temperature gradient is undesirable. For example, the temperature across the top of a catalyst bed should typically not vary by more than one or two degrees which is much less than the hundreds of degrees difference in temperature which may exist at an exchanger exit.
Normally butterfly valves have been used for control of the flow in the external bypass lines because of the size of the ducts. The valves can have many problems including warping of valves bodies because of mechanical and thermal stresses which can cause the valves to jam. In addition, the valves have shaft seals which can leak and allow process gas leakage to the atmosphere where it can cause an environmental nuisance.
Since the unconverted gas in the exchangers is usually colder and does not give a visible plume on leaking, the bypasses have typically been located on the unconverted gas side of the exchanger although there are many cases where the converted gas side might have offered better bypass opportunity.
A further characteristic of exchangers handling gas in contact acid plants is the possibility of tubes with temperatures which lead either to scale formation if the temperature is too hot or to condensation if the metal is too cold. When shell side bypassing takes place around the exchanger the stream of fluid continuing through the exchanger as opposed to the bypass approaches more closely the temperature of the entering tube side fluid and temperatures can be either too high or too low. A bypass operation which offered some protection against this risk would add to plant life and reliability. The external bypass offers no protection against this risk as the whole exchanger is bypassed and the gas is only mixed after leaving the exchanger.
A further feature of exchanger trains found in sulfuric acid plants is that there may be several exchangers in series and external temperature control bypasses may be arranged to bypass several heat exchange steps instead simply of a single step. Such an arrangement often offers very rapid response but saves less in pressure than individual bypasses and it also creates a more severe mixing problem when the multi-step bypass is combined with the main stream.